Lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications. A lithium ion secondary battery generally includes battery components such as a positive electrode, a negative electrode, and a separator that isolates the positive electrode and the negative electrode from one another and prevents short circuiting between the positive and negative electrodes.
In recent years, battery components provided with a porous membrane containing non-conductive particles and a binding material have been used in lithium ion secondary batteries as battery components having improved heat resistance and strength. Specific examples of such battery components include an electrode obtained by forming a porous membrane on an electrode mixed material layer that is provided on a current collector, a separator obtained by forming a porous membrane on a separator substrate, and a separator composed solely by a porous membrane. In order to further improve the performance of lithium ion secondary batteries in which porous membrane-containing battery components such as described above are used, much activity is being focused toward the improvement of porous membranes (for example, refer to PTL 1 and 2).
In one specific example, PTL 1 proposes a technique for improving binding of a porous membrane while inhibiting aggregation of a binding material by using, as the binding material, a carboxy-modified diene-based polymer including from 5 mass % to 85 mass % of an aliphatic conjugated diene monomer unit.
In another example, PTL 2 proposes a technique for improving flexibility of a porous membrane and cycle characteristics of a secondary battery by using a styrene resin and a polymer having a glass transition temperature of no higher than 15° C. as a binding material.